The invention disclosed herein pertains to digital test equipment for verifying the proper operation of digital subsystems that communicate over digital information buses by means of standardized, multi-bit communication words transmitted and/or received over such buses.
Although a test set constructed in accordance with the invention disclosed below has broad application in the testing of digital information exchange systems, it is particularly useful in connection with digital avionics systems and subsystems that are comprised of numerous line replaceable units (LRUs). Each LRU in the avionics system constitutes a replaceable electronics component, and each of these components communicates with others in the overall avionics system by means of transmitting and receiving digital information formatted in standardized multi-bit words that are recognized by the built-in intelligence of each LRU. For example, an LRU may sense the altitude of the aircraft and electronically formulate a digital multi-bit serial word that among other things includes a binary representation of the engineering value of the altitude, whereafter the thusly formulated digital word is transmitted to another LRU which, for example, might be a digital readout on the instrument panel of the cockpit. There, the word is decoded and displayed on the readout in decimal notation or in some other pilot readable form.
While the LRU altimeter is but one example of a line replaceable subsystem, it will be appreciated that the actual avionics system in large sophisticated aircraft of today involve hundreds to thousands of widely varying LRUs which are interconnected by the digital buses of the integrated system to form a complex intelligence system that assists in piloting the aircraft. Because of the complexities of the avionics system when considered as a whole, the aircraft industry has taken steps to standardize the multi-bit digital words that are transmitted and received by the various LRUs so as to facilitate the interchangeability of and maintenance of such components.
One particular aspect of this standardization is to specify that the information transmitted and/or received by at least certain of the LRUs of a typical avionics system shall be in the form of serial, multi-bit communication words, wherein each word includes a data field comprising the engineering or numeric content of the information, a label constituting another subgroup of bits and defining the function of the word (e.g., altitude) and certain other bit groups and single bits representing secondary information. An example of such a standardized digital information word is specified in a publication prepared by Aeronautical Radio, Inc. of Annapolis, Md., entitled "Mark 33 Digital Information Transfer System (DITS) Specification No. 429-3," published Dec. 15, 1979, the contents of which is incorporated herein by reference. The communication word described in Specification 429-3, and known in the industry as the ARINC 429 word, is comprised of 32 bits in a standardized format which, as explained more fully hereinafter, can be used to represent over 300 different functions or parameters generated and/or received by commonly employed LRUs of an avionics system. Altitude was mentioned above and other examples are latitude/longitude, ground speed, magnetic heading, wind speed, runway heading, vertical speed, frequency of radio navigational signals, etc. The information carried in the various 32 bit communication words flows from one LRU port to one or more other LRU ports over a single twisted and preferably shielded pair of wires in which only unidirectional, serial flow of the digital word or words is permitted. Thus, two LRUs that require bidirectional communication will be coupled by at least two separate sets of twisted wire pairs.
Because of the above-mentioned complexities of avionic systems, when the individual LRUs are connected and operating as an integrated system, it is a tedious and formidable task to verify that each LRU is operating properly, and that the various subsystems of two or more coupled LRUs are properly interacting. To a somewhat lesser extent, the testing of stand alone LRUs (disconnected from the system) presents similar difficulties. Available test equipment, including known digital pattern generators and associated displays, have been found lacking in one or more respects in generating, receiving and displaying the test signal information in a format that lends itself to rapid verification of valid operation, and troubleshooting.
Accordingly, it is an object of the invention to provide a test set for use in checking out and troubleshooting LRU based avionics systems having digital communication buses that interconnect the system's LRUs, and that is capable of formulating, transmitting and receiving the inter-LRU digital communication words, and is further capable of displaying the represented communication words in both engineering and binary notation. Still another object of the invention is to provide such a test set in which the generation, transmission and reception of the digital signals is formatted in a standardized multi-bit word that forms the basis of communication between the various LRUs.
Additionally, it is an object of the invention to provide a test set having the above capabilities together with a plurality of individually controllable transmit channels so that two or more communication words can be separately and, if needed, concurrently transmitted for testing the interaction of two or more LRUs in a simulated subsystem.
A further object is to provide a display of the transmitted and received communication words in a concurrent manner so that degradation or other disturbance of the signals that represent the word can be quickly and easily isolated to facilitate troubleshooting.
Also, it is an object to provide such a test set having a monitoring capability for rapidly identifying one or more particular communication words that are present on a communication bus and are received by the test set.
A further object is to provide such a test set having the capability of concurrently transmitting a communication word on one bus and at the same time receiving a communication word on another bus so that an LRU or a subsystem of LRUs can be stimulated by an artificially generated and transmitted word, and the response to such stimulation monitored in the form of the received communication word.
One additional object is to provide a test set having the foregoing capabilities in conjunction with a keyboard entry that permits the formulation of a test transmit word in either engineering or binary form.